Sorry to hear about Mauri! It is nice though to hear from someone in Finland with this problem, as I've also had quite a hard time getting information/treatment for Sammy (also has SM *most likely*).
I'm really curious what neurologist you went to? I only heard about one neurologist in Finland -in Aisti - who wouldn't give me any advice unless Sammy got the MRI. Unfortunately I'm not able to get afford the MRI as I'm a student (though I've begun to appy for full time jobs again, so I can get him better treatment). How much was your MRI?
The vet I'm seeing, in Jyväskylä, had only ever prescribed Metacam for SM dogs - which wasn't enough for Sammy. I printed out Claire Rusbridge's treatment pdf, and the vet has agreed to follow that. It recommended starting on 100mg gabapentin twice a day. The pharmacy ordered a liquid form of gabapentin for dogs for me. They had only ever heard of it for epilepsy, so I am really relying on the information I've found here (see also smcavalier.com). The gabapentin's made a BIG difference, although the dose isn't quite enough as he still scratches sometimes. His symptoms were quite severe. I'm surprised to hear that Mauri was started on such a large dose!
Could I ask where you get your gabapentin? It could be that the neurologist has started you on such a high dose because the lowest capsules available in Finland are 300mg. However, there is a liquid form for dogs (a bit more expensive though), and it is also possible to divide the capsules yourself into smaller doses. Pauline, on this site, sent me a video of how to do this.
Best wishes for Mauri,
& Samwise (7 year old ruby male)